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POCan you throw within a conditional expression? (was: How can bounds-checking be extended to multiple dimensions?)
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Note: I solved the original problem by realizing a completely different one. See the addendum for the new actual problem, but you can read the previous part for context. This is an extension of one of my <a href="https://stackoverflow.com/q/10171525/1010226" title="How can one make a variadic-based chained-indexing function?">previous posts</a>. I made a container class based on that answer: <pre><code>template < typename T, unsigned N0, unsigned ...N > struct array_md { // There's a class template specialization with no extents. // Imagine the various N... components are bracket-enclosed instead // of comma-separated. And if "N..." is empty, then we just have "T" // as the "direct_element_type". (I actually use recursive class // definitions.) using direct_element_type = T[N...]; using data_type = direct_element_type[ N0 ]; template < typename ...Indices > auto operator ()( Indices &&...i ) noexcept( !indexing_result<data_type &, Indices...>::can_throw ) -> typename indexing_result<data_type &, Indices...>::type { return slice(data, static_cast<Indices &&>( i )...); } template < typename ...Indices > constexpr auto operator ()( Indices &&...i ) const noexcept( !indexing_result<data_type &, Indices...>::can_throw ) -> typename indexing_result<data_type &, Indices...>::type { return slice(data, static_cast<Indices &&>( i )...); } data_type data; }; </code></pre> I'm trying to make a version of <code>at</code> for this container. I figured I'll just make a version of <code>slice</code> that takes an exception object. Unlike the general <code>slice</code>, my <code>checked_slice</code> has to take in a built-in array object, since pointers and class types (with <code>operator []</code>) don't have a (standard) way to give me bounds. <pre><code>template < typename E, typename T > inline constexpr auto checked_slice( E &&, T &&t ) noexcept -> T && { return static_cast<T &&>(t); } template < typename E, typename T, std::size_t N, typename ...V > inline constexpr auto checked_slice( E &&e, T (&t)[N], std::size_t u, V &&...v ) -> typename remove_some_extents<T[N], 1u + sizeof...(V)>::type & { return u < N ? checked_slice( static_cast<E &&>(e), static_cast<T &>(t[ u ]), static_cast<V &&>(v)... ) : throw static_cast<E &&>( e ); } template < typename E, typename T, std::size_t N, typename ...V > inline constexpr auto checked_slice( E &&e, T (&&t)[N], std::size_t u, V &&...v ) -> typename remove_some_extents<T[N], 1u + sizeof...(V)>::type && { return u < N ? checked_slice( static_cast<E &&>(e),static_cast<T &&>(t[ u ]), static_cast<V &&>(v)... ) : throw static_cast<E &&>( e ); } </code></pre> (The <code>remove_some_extents</code> does what it says, instead of just one or all the C++11 standard library gives you.) When I put this into my <code>at</code>: <pre><code>template < typename T, unsigned N0, unsigned ...N > struct array_md { //... template < typename ...Indices > auto at( Indices &&...i ) -> typename remove_some_extents<data_type, sizeof...( Indices )>::type & { return checked_slice(std::out_of_range{ "Index out of bounds" }, data, static_cast<Indices &&>( i )...); } template < typename ...Indices > constexpr auto at( Indices &&...i ) const -> typename remove_some_extents<data_type,sizeof...( Indices )>::type const & { return checked_slice(std::out_of_range{ "Index out of bounds" }, data, static_cast<Indices &&>( i )...); } //... }; </code></pre> I get errors related to array-to-pointer decay! (I'm using TDC-GCC 4.7.1 that's bundled with CodeBlocks 12.11 for Windows-8 Pro 32-bit.) <pre><code>In file included from container/array_md.hpp:36:0, from test\arraymd_test.cpp:15: utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const int; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const int; std::size_t = unsigned int]': container/array_md.hpp:284:112: required from 'constexpr const typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) const [with Indices = {int}; T = int; unsigned int M = 2u; unsigned int ...N = {}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = int]' test\arraymd_test.cpp:224:1: required from here utility/slice.hpp:141:10: warning: returning reference to temporary [enabled by default] utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [2][6]; std::size_t = unsigned int]': container/array_md.hpp:284:112: required from 'constexpr const typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) const [with Indices = {int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [2][6]]' test\arraymd_test.cpp:238:1: required from here utility/slice.hpp:141:10: error: invalid initialization of reference of type 'const char (&)[2][6]' from expression of type 'const char (*)[6]' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [2][6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': utility/slice.hpp:141:10: required from 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]' container/array_md.hpp:284:112: required from 'constexpr const typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) const [with Indices = {int, int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:239:1: required from here utility/slice.hpp:141:10: error: invalid initialization of reference of type 'const char (&)[6]' from expression of type 'const char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': container/array_md.hpp:284:112: required from 'constexpr const typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) const [with Indices = {int, int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:239:1: required from here utility/slice.hpp:141:10: error: invalid initialization of reference of type 'const char (&)[6]' from expression of type 'const char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {unsigned int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': container/array_md.hpp:284:112: required from 'constexpr const typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) const [with Indices = {int, unsigned int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:261:5: required from here utility/slice.hpp:141:10: error: invalid initialization of reference of type 'const char (&)[6]' from expression of type 'const char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {unsigned int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = int; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = int; std::size_t = unsigned int]': container/array_md.hpp:274:112: required from 'typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) [with Indices = {int}; T = int; unsigned int M = 2u; unsigned int ...N = {}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = int]' test\arraymd_test.cpp:220:1: required from here utility/slice.hpp:141:10: error: invalid initialization of non-const reference of type 'remove_some_extents<int [2], 1u>::type& {aka int&}' from an rvalue of type 'int' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = int; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = int; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [2][6]; std::size_t = unsigned int]': container/array_md.hpp:274:112: required from 'typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) [with Indices = {int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [2][6]]' test\arraymd_test.cpp:234:1: required from here utility/slice.hpp:141:10: error: invalid initialization of non-const reference of type 'char (&)[2][6]' from an rvalue of type 'char (*)[6]' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [2][6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': utility/slice.hpp:141:10: required from 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]' container/array_md.hpp:274:112: required from 'typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) [with Indices = {int, int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:235:1: required from here utility/slice.hpp:141:10: error: invalid initialization of non-const reference of type 'char (&)[6]' from an rvalue of type 'char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': container/array_md.hpp:274:112: required from 'typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) [with Indices = {int, int}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:235:1: required from here utility/slice.hpp:141:10: error: invalid initialization of non-const reference of type 'char (&)[6]' from an rvalue of type 'char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In instantiation of 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {long double}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': container/array_md.hpp:274:112: required from 'typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type& container::array_md<T, M, N ...>::at(Indices&& ...) [with Indices = {int, long double}; T = char [6]; unsigned int M = 2u; unsigned int ...N = {2u}; typename remove_some_extents<typename container::array_md<T, N ...>::data_type [M], sizeof (Indices ...)>::type = char [6]]' test\arraymd_test.cpp:260:5: required from here utility/slice.hpp:141:10: error: invalid initialization of non-const reference of type 'char (&)[6]' from an rvalue of type 'char*' utility/slice.hpp:142:1: error: body of constexpr function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {long double}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]' not a return-statement utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {unsigned int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = const char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = const char [2][6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {long double}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {int}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = char [2][6]; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = char [2][6]; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] utility/slice.hpp: In function 'constexpr typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type& checked_slice(E&&, T (&)[N], std::size_t, V&& ...) [with E = std::out_of_range; T = int; unsigned int N = 2u; V = {}; typename remove_some_extents<T [N], (1u + sizeof (V ...))>::type = int; std::size_t = unsigned int]': utility/slice.hpp:142:1: warning: control reaches end of non-void function [-Wreturn-type] </code></pre> I thought using array references cancels array-to-pointer decay. Is this a GCC bug, or am I messing up somewhere? For "slice.hpp", Line 141, Col. 10 is the end of the statement in the (l-value version of) <code>checked_slice</code> function, while L142C1 is the function's ending bracket. In "array_md.hpp", L284C112 and L274C112 are return (and sole) statements of the <code>at</code> functions, <code>const</code> and non-const respectively. The column is at the "i" inside the <code>static_cast</code>. By the way, here's <code>remove_some_extents</code>: <pre><code>// Forward declaration template < typename Array, std::size_t Count > struct remove_some_extents; // Case with indefinite array but no extents to strip template < typename T > struct remove_some_extents< T[], 0u > { typedef T type[]; }; // Case with definite array but no extents to strip template < typename T, std::size_t N > struct remove_some_extents< T[N], 0u > { typedef T type[N]; }; // Case with non-array type but no extents to strip template < typename T > struct remove_some_extents< T, 0u > { typedef T type; }; // Case with indefinite array and extents to strip template < typename T, std::size_t L > struct remove_some_extents< T[], L > { typedef typename remove_some_extents<T, L - 1u>::type type; }; // Case with definite array and extents to strip template < typename T, std::size_t N, std::size_t L > struct remove_some_extents< T[N], L > { typedef typename remove_some_extents<T, L - 1u>::type type; }; // Right now, non-array type with non-zero strip count should give an error. </code></pre> Thanks. Edit: Added the base-case and r-value overloads for <code>checked_slice</code>. Addendum: I got something that works, but I don't know why the old way didn't work. I first commented out the r-value overload for <code>checked_slice</code>, to reduce the variables I have to work with. I then made a version of <code>checked_slice</code> that works for standard containers, but you don't need to see it because it didn't help. (And I commented it out to make sure it didn't make a difference.) I changed the regular version to: <pre><code>template < typename E, typename T, std::size_t N, typename ...V > inline constexpr auto checked_slice( E &&e, T (&t)[N], std::size_t u, V &&...v ) -> typename remove_some_extents<T[N], 1u + sizeof...(V)>::type & { return checked_slice( static_cast<E &&>(e), static_cast<T &>(t[ u ]), static_cast<V &&>(v)... ); } </code></pre> i.e. I removed the actual test and the throw-on-fail parts, and the code worked! The problem didn't seem to be the indexing, but the throw and/or conditional! Sure enough, when I changed it to: <pre><code>template < typename E, typename T, std::size_t N, typename ...V > inline auto checked_slice( E &&e, T (&t)[N], std::size_t u, V &&...v ) -> typename remove_some_extents<T[N], 1u + sizeof...(V)>::type & { if ( u < N ) return checked_slice(static_cast<E &&>(e),t[u],static_cast<V &&>(v)...); else throw e; } </code></pre> It still works! What gives, I thought it was OK to use a throw statement as one of the action parts of a conditional expression? Is it a bug in my compiler? I may have to give up and separate the bounds-check to a pure-function and throw-on-fail within the <code>at</code> method.
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<c++11><multidimensional-array><conditional-operator><indexoutofboundsexception><throw>
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Can you throw within a conditional expression? (was: How can bounds-checking be extended to multiple dimensions?)
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